The use and development of bracing grids in fuel subassemblies goes back over a long period. For example GP-PS No. 1,052,777 shows a grid having dimpled circular tubes to define unit cells to locate fuel pins and also has other non-circular un-dimpled tubes mating with the circular tubes to space them apart. This is a complicated structure, it has low compliance, it requires a multiplicity of tube sections and it is time consuming and costly to fabricate. In any event the pin locating unit cells are not closely spaced, and cannot be closely spaced, as required by the high rating of fast reactors.
In a further prior art disclosure, namely GB-PS No. 1,386,424 the concept of a "joggled" strip grid is introduced. The strip, in the course of its length, defines the dimpled walls of many cells but never itself defines a complete unit cell. Each cell in the grid is defined by a second level of joggled strips which cooperate with those in the first level. This introduces complexity and problems of matching two sets of strips at different levels with the dimples correctly placed and each set of strips introduced entry and exit power losses in the flow of coolant through the grids.
A single level, dimpled, joggled strip form of grid has however been used in which the cells in the grid are defined by strip material confined to a single level. There has been no known publication of this grid and accordingly it is described below with reference to the drawings. As described, the grid has edge cells which are formed in part by the joggled strips and in part by shaped supplementary strips. This grid has an inherent mechanical weakness and constructional problems which the present invention seeks to overcome whilst still retaining the advantages which are inherent with joggled strip.